Diffusion coating of metals

ABSTRACT

AN IMPROVED METHOD FOR DIFFUSION C OATING A METAL, E.G. ALUMINUM, INTO THE SURFACE OF A BASE METAL ARTICLE, E.G. CHROMIUM-CONTAINING STEEL, BY THE PACK CEMENTATION PROCESS IS PROVIDED WHICH ENABLES THE USE OF CEMENTATION TEMPERATURES AS LOW AS 750* TO 775*F. IN ESSENCE THE METHOD COMPRISES, PROVIDING A CEMENTATION PACK COMPRISED OF PARTICULATE COATING METAL (E.G. ALUMINUM), THE CEMENTATION PACK HAVING UNIFORMLY MIXED THEREWITH AN AMOUNT OF A DRY CLEANSING AND TRANSPORT-INDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF METAL HALIDES AND IODINE (SUCH AS ALCL3) EFFECTIVE TO PROMOTE THE DEPOSITION OF SAID COATING METAL AT AN ELEVATED METAL COATING AND DIFFUSION TEMPERATURE, THE CLEANSING AND TRANSPORT-INDUCING AGENT BEING CHARACTERIZED BY SUBSTANTIAL VAPOR PRESSURE WHEN HEATED TO ABOVE AMBIENT TEMPERATURE AND BELOW THE METAL COATING AND DIFFUSION TEMPERATURE. THE BASE METAL ARTICLE IS COMPLETELY EMBEDDED INTO THE CEMENTATION PACK CONFINED IN A CLOSED RETORT AND THE PACK WITH THE EMBEDDED ARTICLE THEN SUBJECTED TO A HEATING CYCLE BY HEATING THE PACK TO A METAL COATING AND DIFFUSION TEMPERATURE AT A RATE CHARACTERIZED BY AN ENDOTHERMIC ARREST AT A TEMPERATURE BELOW SAID METAL COATING AND DIFFUSION TEMPERATURE, WHEREBY SUBSTANTIAL OUTGASSING OCCURS DURING SSAID ENDOTHERMIC ARREST, AND THE HEATING TO AND AT THE METAL COATING AND DIFFUSION TEMPERATURE BEING THEN CONTINUED UNTIL THE DESIRED COATING THICKNESS HAS BEEN OBTAINED.

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United States Patent US. Cl. 117-107.2 P 7 Claims ABSTRACT OF THE DISCLOSURE An improved method for diffusion coating a metal, e.g. aluminum, into the surface of a base metal article, e.g. chromium-containing steel, by the pack cementation process is provided which enables the use of cementation temperatures as low as 750 to 775 F. In essence the method comprises, providing a cementation pack comprised of particulate coating metal (e.g. aluminum), the cementation pack having uniformly mixed therewith an amount of a dry cleansing and transport-inducing agent selected from the group consisting of metal halides and iodine (such as AlCl effective to promote the deposition of said coating metal at an elevated metal coating and diffusion temperature, the cleansing and transport-inducing agent being characterized by substantial vapor pressure when heated to above ambient temperature and below the metal coating and diffusion temperature. The base metal article is completely embedded into the cementation pack confined in a closed retort and the pack with the embedded article then subjected to a heating cycle by heating the pack to a metal coating and diffusion temperature at a rate characterized by an endothermic arrest at a temperature below said metal coating and diffusion temperature, whereby substantial outgassing occurs during said endothermic arrest, and the heating to and at the metal coating and diffusion tempearture being then continued until the desired coating thickness has been obtained.

This application is a continuation-in-part of US. application Ser. No. 878,596, filed Nov. 21, 1969 now abandoned.

This invention relates to the pack cementation diffusion coating of metals into the surface of metal articles, such as chromium-containing steels, embedded and heated in a diffusion coating pack, and more particularly, to techniques and compositions whereby a cleansing and transport-inducing agent is included in the pack to promote the diffusion of the coating metal from the pack into the surface of the articles being coated, the diffusion being greatly facilitated at a particular coating temperature and/or satisfactory diffusion coatings being obtained at coating temperatures lower than those normally required with conventional diffusion coating processes.

THE FIELD OF THE INVENTION The diffusion coating processes to which the invention relates include those in which the metal articles to be coated and protected against corrosion, e.g. turbine vanes, shrouds and the like, are embedded in a powder pack which contains the coating metal (e.g. aluminum, chromium, etc.), usually an inert filler (such as powdered alumina), and an energizer component (such as halide,

3,764,373 Patented Oct. 9, 1973 "ice e.g. NH I) for aiding the transport or transfer of the coating metal from the powdered pack to the surface of the articles to be coated. The pack with the embedded articles is heated in a closed retort (usually in the partial absence of oxygen) to relatively high temperatures for a sufficient time to produce the diffusion of the coating metal into the surface of the articles to a desired depth. Such processes are employed to enhance the oxidation resistance of the articles at elevated temperatures and to enhance erosion and corrosion resistance for a variety of purposes and uses. The conventional pack cementation diffusion coating techniques may require that the coating step 'be prolonged for many hours, or for even more than a day, at relatively high temperatures, usually in excess of 1200 F. and as high as 1800 F. to 2000 F., depending upon the particular metal substrate, the thickness of coating desired and other characteristics.

THE PROBLEM CONFRONTING THE ART There are a variety of metals and alloys the physical or mechanical properties of which tend to be altered or adversely affected when they are heated above, for example, 1000 F. for any reason. Examples of such metals or al loys are precipitation hardenable stainless steels, such as 17-4PH (17% Cr, 4% Ni, 3% Cu, smaller amounts of Co, Mn, Si and the balance essentially iron), type 410 (11.5 to 13.5% Cr, 1% Si max., 1% Mn max., 0.15 carbon max. and the balance essentially iron), and AMS 5'616 (13% Cr, 2% Ni, 3% W and the balance essentially iron), among many others. The tempering temperature for the foregoing type of conventional precipitation hardenable steels may range from 875 F. to 1150 F. As will be obviously apparent, diffusion coating temperatures of over 1200 F. would have an adverse affect on the hardness of the steels to the extent of altering the physical properties and to the extent of the rendering the arciles unsuitable for use.

As a more specific illustration of commercial articles in which maintenance of physical properties during environmental use is an important and necessary requisite, attention is directed to certain components of the compressor portions of jet aircraft engines. These components are made of high strength steels so that they can withstand the tremendous mechanical stresses resulting from centrifugal force, thermal shock, and vibrations at temperatures which may range as high as about 850 F. and not exceeding 900 F. While such temperatures are not considered high when compared to the elevated temperatures (1600 F. to 1800" F.) to which superalloy aircraft components are subjected, nevertheless the steel compressor components must be coated in order to protect the substrate metal in highly saline environments which prevail when low flying aircraft operate at or near the seashore, the atmosphere of which may also include substantial amounts of sand and coral dust which tend to be highly erosive. Thus, by providing a surface diffusion coating of aluminum which produces an iron-aluminum intermetallic compound on the steel substrate of the compressor component which coating is galvanically sacrificial and at the same time resistant to dust erosion and/or saline corrosion, the component is capable of being used for relatively prolonged periods of time, provided that the original physical properties have not been substantially adversely affected by the coating process. This is particularly important in the case of those components which are shot peened in order to enhance their physical properties. As is well known, shot peening produces residual compressive loading into the metal surface which improves the high cycle fatigue performance of the components, and particularly enables the recovery of fatigue properties of previously corrosion damaged hardware.

Now shot peening is effective only if subsequent processing temperatures do not relieve the required level of compressive stresses which may penetrate into the surface to a depth of as much as ten thousandths of an inch.

One method which has been proposed for lowering the metal coating and diffusion temperatures to desirable levels comprises mixing with the powdered cementation pack an accelerator component, preferably a metal which is substantially volatile at the coating temperature, such as cadmium, lead, tin and zinc. While the accelerator metal substantially facilitated diffusion coating at desirably lower temperature, for example, temperatures in the neighborhood of about 900 F, it had certain drawbacks. The metals cadmium and lead presented the problem of toxicity and thus extreme caution had to be taken to avoid fumes from reaching the working environment. Another drawback resided in the fact that the accelerator metal, e.g. zinc, would tend to deposit out with the coating metal, despite the fact that that the coating material comprised substantially the coating. This is undesirable where optimum resistance to corrosion and fatigue is required. Thus, inhomogeniety in the ultimate coating had to be avoided.

Another problem that presented itself, and this was true of the conventional techniques as well, and that is the fact that the particulate coating metal generally had to be extremely fine to assure the production of satisfactory coatings from both the chemistry and thickness viewpoints.

However, the use of fine aluminum powder of mesh size minus 325 mesh with an average particle size of microns carried with it an explosion hazard because of the propensity of such powders of forming dust clouds when handled during packing and unpacking operations and which therefore tended to oxidize spontaneously and exothermically and/or ignite. The US. Bureau of Mines rates such explosibility hazards as severe. The concentration of the aluminum powder that produces such hazards is indicated as being approximately 0.04 oz. per cubic foot of air. Reducing the metal content the cemen tation pack does not eliminate the problem. The explosibility hazard is minimal when the particle size is greater than about 80 or 100 microns, e.g. greater than 200 or, more preferably 150 mesh. However, those skilled in the art prefer to use fine particles of high packing density to assure enhanced coating surface finish and improved throwing power. On the other hand, coarse metal coating particles does not provide such advantages but, on the contrary, the coating tends to be unsatisfactory.

Summarizing the problem, it thus became apparent that a method was required for producing a galvanically sacrificial coating on steel substrates at temperatures below 850 F. where compressive stresses introduced into the surface of the substrate by shot peening could be retained and also without adversely affecting the particular temper of the steel.

It would be particularly desirable to provide a method which could utilize coarse non-explosive aluminum powder (e.g. plus 150 mesh) and provide an aluminum iron intermetallic coating which would be sacrificial to the basis material and yet would not degrade in any way the fatigue properties of the basis metal. Obviously, to achieve the foregoing, the system would have to be one that will produce a clean coating-base metal interface.

With the foregoing objects in mind, the invention will now be described in more detail, and other objects and advantages will be apparent from the following disclosure and the appended drawings, wherein:

FIG. 1 is a plot showing the endurance limit in corroded and shot peened specimens after coating at 795 F. for 42 hours using a cementation pack provided by the invention containing aluminum of 60+ mesh screen size and 20% alumina, the pack containing about 3% by Weight of dry aluminum chloride intimately mixed therewith;

FIGS. 2, 2A and 2B are electron microprobe compositional profile charts of a particular basis metal such as AMS 5616 steel treated in accordance with the invention showing the aluminum profile in the coating relative to the constituents of the steel substrate, the microprobe trace being determined across the coating and into the steel substrate, the aluminum coating being produced at 795 F. for 38 hours;

FIGS. 3, 3A and 3B is a similar electron microprobe profile chart obtained on aluminum coated AMS 5616 steel produced in accordance with the invention at 875 F. showing an increase in the amount of aluminum by using a metal coating and diffusion temperature for 28 hours at 875 F.; and

FIG. 4 is a time-temperature profile chart illustrating the effect of a relatively fast and a relatively slow heat up cycle on the production of aluminum coatings.

STATEMENT OF THE INVENTION In its more preferred aspect, we provide an improved method for diffusion coating aluminum into the surface of chromium-containing steels by pack cementation wherein the steel article is embedded in a cementation pack and heated to an elevated metal coating and diffusion temperature to effect diffusion of aluminum in the steel article, the diffusion temperature being advantageously below (but not limited thereto) that temperature which would be detrimental to the physical properties of the steel. The invention employs a cementation pack comprised of particulate aluminum, preferably though not limited to coarse aluminum powder, the cementation pack having uniformly mixed therewith an amount of dry aluminum chloride powder as a cleansing and transport-inducing agent effective to promote the deposition and diffusion of aluminum onto the steel article at the desirable metal coating and diffusion temperature. The aluminum chloride is characterized by substantial vapor pressure when heated to above ambient temperature and below the metal coating and diffusion temperature. The steel article is completely embedded into the cementation pack confined in a closed retort and the assembly subjected to a timetemperature heating cycle in which the pack is brought up from ambient temperature to the desired metal coating and diffusion temperature, preferably below the temperature which adversely affects the physical properties of steel, at a rate characterized by an endothermic arrest at a temperature below the metal coating and diffusion temperature, whereby substantial outgassing occurs during the period of endothermic arrest, the heating being continued to and at the preferred temperature until the desired aluminum coating thickness has been obtained.

The aforementioned preferred embodiment provides a pack cementation process with markedly improved activity at substantially lower temperatures than hereto possible by known methods. In essence, the process greatly decreases the incubation period which normally prevails in prior processes due to contamination of the metal substrate by surface oxides, residues, and the like, and due also to the surface contamination (e.g. oxides) of the particulate coating metal itself. Thus, by employing the invention, the rate of transport of the coating metal at a given temperature from the pack source is greatly enhanced coincidentally with the elimination of the incubation period which is related to the removal of surface oxides, residues or other contaminants from both the surface of the substrate and the surfaces of the particulate coating metal in the pack, which contaminants on the coating powder othewise inhibit the transport of the coating metal to the metal substrate, the contaminants on the metal substrate inhibiting diffusion.

It is to this extent that the promoter added to the cementation pack is referred to as the cleansing and transport-inducing agent in that it provides a two-fold function in a one-step operation depending upon the heating rate employed. It is a cleansing agent in that during the endothermic arrest, large amounts of a halogen-containing gas are evolved at substantially below the metal coating and diffusion temperature which cleanses the surfaces of the contaminants and thereby enhances the transport rate of the aluminum from the pack to the substrate and, when the desired diffusion temperature is reached, greatly enhances the diffusion rate of the aluminum into the already cleaned substrate of the steel article. Thus, in effect, the invention provides a self-cleaning pack composition which cleans the aluminum particles of oxide, organics etc., as well as clean (flux) the surface of the steel of Fe O and/or alloy oxides such as Cr O etc. Cleaning of pack metal particles allows the use of larger particles of, for example, aluminum since the smaller available surface area of large particles relative to small particles is over compensated by the available active surface produced by the cleansing and transportinducing agent (e.g. AlCl The promoter is a transport-inducing agent in that it triggers and sustains the cycle of metal transport from the pack to the substrate. That is to say, the promoter, which may comprise other metal halides or even iodine, must be compatible to the extent of reacting with the clean aluminum and produce gaseous aluminum compounds which are then transported to the clean metal substrate for diffusion thereinto.

Thus, in its more broad aspects, the invention is applicable to other metal substrates besides steel or ferrous alloys and to other coating metals besides aluminum, such as zinc, antimony, cadimum, etc. Broadly stated, the invention comprises providing a cementation pack comprised of particulate coating metal having mixed intimately therewith an amount of a dry cleansing and transportinducing agent selected from the group consisting of metal halides and iodine effective to promote the deposition of the coating metal at a desired elevated metal coating and diffusion temperature, the agent being characterized by substantial vapor pressure when heated to above ambient temperature and below the desired metal coating and diffusion temperature. The metal article is completely embedded into the cementation pack confined in a closed retort and the assembly then subjected to a heating cycle by heating the pack to the desired metal coating and diffusion temperature at a rate characterized by an endothermic arrest at a temperature below the metal coating and diffusion temperature, whereby substantial outgassing occurs during the endothermic arrest and the heating of the pack to and at the metal coating and diffusion temperature continued until the desired coating thickness is obtained.

We prefer that the metal component of the metal halide constituting the cleansing and transport-inducing agent be the same as the coating metal in the pack; however, the metal component of the agent need not be the same, as it may be desirable in some instances to deposit an alloy coating, that is the metal component from the cleansing and transport-inducing agent together with a different coating metal from the pack.

As stated hereinabove, the process provided by the invention is particularly advantageous in that relatively coarse particles of coating metal may be employed which avoid explosion hazards. However, it is not to be construed that the invention cannot be employed with finer powders, such as 325 mesh aluminum, provided the necessary precautions are taken to avoid conditions which may cause explosions. Thus, while the pack has been stated as being essentially comprised of particulate coating metal, it is to be understood that the pack may comprise the coating metal itself (particularly where coarse particles are employed), or the pack may be a blend of the coating metal in major amounts with particulate inert material. The coating material as the major constituent of the pack may range from about 60% to 100% by weight and the balance essentially the inert material. Examples of inert material are alumina, thoria, calcia, zirconia and other stable and inert refractory oxides and mixtures thereof.

While any particle size may be employed in the pack, we find it advantageous for our purposes to work at particle sizes larger than 200 mesh, more preferably, above 150 mesh, and generally from about 60 mesh to +150 mesh. The inert material may have the same particle size range as the coating metal and, generally speaking, we prefer that both the coating metal and the inert material be coarse because of ease of handling, mixing, and the like.

DETAILS OF THE INVENTION The more preferred aspects of the invention will be discussed relative to the use of aluminum chloride as the cleansing and transport-inducing agent in the production of aluminum coatings on a precipitation hardenable stainless steel, it being understood that what is said about aluminum coatings will hold generally for other metals, keeping in mind the usual variations that may have to be made to suit a particular coating metal.

Tests conducted on a shot peened steel specimen of AMS 5616 steel (13% Cr, 2% Ni, 3% W and the balance essentially iron) indicated that the invention was particularly applicable to such steels without substantially adversely affecting the compressive stresses induced by shot peening, thus maintaining the fatigue resistance level of the part and in some instances augmenting it to still higher levels because of the exceptionally good quality coating deposited on the steel part. In addition, the hardness of the part was maintained at relatively the same level.

One testing using aluminum chloride as the cleansing and transport-inducing agent revealed that a 795 F. for 42 hours coating cycle provided retention of sufiicient compressive stresses to sustain the same endurance limit as that shown by the compressively stressed specimens with no thermal treatment. Typical results are shown in FIG. 1. The desired properties of the coating were also obtained on type 410 stainless steel using the same time and temperature cycle.

Referring to FIG. 1, a previously corroded test specimen (that is a specimen which had been subjected to a salt spray test) exhibited the fatigue rating (about 50 k.s.i.) illustrated by curve C. However, when the corroded specimen was shot peened, its fatigue rating was markedly improved to the level (about k.s.i.) shown by curve B. On the other hand, when the shot peened steel part was coated with aluminum in accordance with the invention, the quality of the coating as well as the substrate upon which it was deposited was so markedly improved that the fatigue rating was augmented to the still higher level of about k.s.i. (Note curve A.)

In carrying out the process, it was found that the aluminum content of the pack could be varied from to 60% and, when less than 100% metal content is em ployed in the pack, aluminum oxide is selected as the inert material. Preferably, the oxide has a mesh size which is similar to the metal particles, for example, 60 to +140, but it has been found that the --325 mesh aluminum oxide can also be used. The aluminum chloride is added to the pack in the range of about 1 to 3%, although up to about 5% can be employed. Processing of the coating compound is preferably carried out under humidity control conditions with a relative humidity less than 60% or, more advantageously, 45% or lower employed to prevent undesirable reaction with moisture. Freshly blended powder is preferably burnt out prior to being used for the coating cycle.

The burn-out cycle is accomplished by heating the pack comprising aluminum, aluminum oxide, aluminum chloride powders in a retort for times sufficient to expel any residual moisture in the aluminum or aluminum oxide. This drying may be effected at about 795 to 900 F. for times of 12 hours or longer. The burnt out powder or a previously used batch of powder can then be used for coating by simply adding dry aluminum chloride to it. Metal retorts of steel or aluminum are used to hold the powder and the parts are completely embedded in the coating compound. The retort is covered with a lid and a gasket of asbestos or multiple layers of aluminum foil is used to effect a seal between the lid and the retort body, the seal being suflicient to keep out air but permit the outgassing of generated halogen-containing gases. Shot peened hardware or items requiring minimal coating buildup can be processed at 795 F. for times of 38 hours. The chemical profile (electron microprobe trace) of a 13% Cr steel (AMS 5616) coated in this manner is shown in FIGS. 2, 2A and 2B. In cases where a thicker case depth is sought and no consideration need be given to shot peening, a satisfactory coating cycle has been found to be 875 F. for 28 hours. The resulting chemistry of the iron-aluminum intermetallic as determined by the electron microprobe is shown in FIGS. 3, 3A and 3B.

Upon completion of the coating cycle, the retorts are removed and cooled in an area free of moisture. When the coated parts are unpacked, any adhering powder is immediately blown ofi' or rinsed off with water to prevent the possibility of reactions between the compound and the coating in the presence of moisture.

Referring to the chemical profile of FIGS. 2, 2A and 23, it will be noted that a coating thickness of about three-quarters of a thousandth of an inch containing a peak amount of about 54.6% aluminum is produced at the relatively low temperature of about 795 F. for 38 hours. In preparing the specimen for the electron microprobe analysis, a cross-sectional piece of the coating and substrate is cut from the specimen and the coated face side plated with nickel. The electron beam trace is made over a span of 0.001 inch from the substrate side to the nickel plate interface as shown graphically at the top of the drawing. As will be noted, the chromium level in the coating has dropped from 14.5% to about 6.8% chromium due to the diluting effect of the diffused aluminum.

In FIG. 2A, the iron level in the same piece has dropped from 82.3% to 39.8% iron in the coating. In FIG. 2B, the coating retained the level of 2% nickel present in the substrate steel.

As stated hereinabove, higher levels of aluminum are obtainable at higher temperatures as illustrated in FIGS. 3, 3A and 3B. As will be noted, when the coating was produced on AMS 5616 steel at 875 F. for 28 hours, a coating thickness of approximately 0.001 inch was obtained containing a peak amount of aluminum of about 60.3%, the chromium content in the steel of 14.3% being diluted in the coating to a level of about 5.8% chromium, the nickel being diluted from 1.9% to 1% (FIG. 3A), and the iron from 82.6% to 34.8% (FIG. 3B). The coating obtained was of particular high quality, considering that the temperature of 875 F. employed is still substantially below the temperatures employed in the more conventional cementation packs. This markedly improved result is due to the self-cleaning characteristics of the pack.

As stated hereinbefore, by assuring an endothermic arrest during the heating up cycle, whereby outgassing of a halogen-containing gas occurs below the metal coating and diffusion temperature, the coating metal particles and the steel substrate are cleaned so that when the desired temperature is reached, efficient transport of the coating metal and diffusion into the substrate occurs with improved ease. By endothermic arrest is meant that condition that obtains when the rate of temperature rise is arrested or decreased due to the absorption of heat energy by virtue of the latent heat of vaporization of the cleansing and transport-inducing agent, e.g. aluminum chloride.

When aluminum chloride is used as the active agent, the following system behavior has been observed. As the dry mixture of aluminum, aluminum oxide and aluminum chloride is heated to the desired coating temperature, a pronounced outgassing is observed at 350 F. This closely corresponds to the melting point or the sublimation temperature of AlCl which are 374.4 F. and 356.4 K, respectively. During outgassing, vast quantities of a halogen-containing gaseous product under considerable pressure are released from the retort through the gasket. Pressure within the retort, which is not designed specifically to contain the gas, has been measured as high as five pounds. This will be understood when it is realized that pressures of 2 atmospheres at 370 F. have been reported as the vapor pressure of aluminum chloride. Continued heating results in a decrease and finally, an absence of this gaseous product which has been determined to be obstensibly hydrogen chloride.

During the period when vapors are being expelled, a very fine residue is observed to form on the outer walls of the retort. This has been sampled and determined by spectrographic examination to be predominately aluminum or a compound of aluminum plus other trace elements. The insolubility of the product in acids, and subsequent X-ray diffraction studies have established that the aluminum compound is most probably aluminum oxide. A complete spectrographic analysis of the residue appears to indicate that the trace elements may be metal removed from the metal substrate due to an etching action of aluminum chloride as is indicated by the following comparison of the trace elements found in the residue and actual elements present in the AMS 5616 substrate.

Trace Elements Found 'by Spectrographic Elements present Analysis in AMS 5616 Manganese Manganese Silicon Silicon Phosphorous Sulphur Chromium Chromium Nickel Nickel Molybdenum Tungsten Aluminum Aluminum Copper Copper Tin Tin Iron Iron This would probably explain why the fatigue resistance of the shot peened steel part is further improved as shown in the plot of FIG. 1. It is believed that the aluminum chloride aids chemically in removing discontinuities, small notches, and the like from the surface of the substrate which can and are known to produce adverse effects on the fatigue life of the steel part in actual service.

The endothermic arrest E in the case of aluminum chloride is shown in FIG. 4 to occur at approximately 350 F. from about 1 to 2 hours from the initiation of the heating cycle, another arrest occurring at about 735 F., both arrests occurring below the metal coating and diffusion temperature. By advantageously starting the heating cycle by placing the charged retort in a cold furnace, the desired heating rate is assured for taking full advantage of the endothermic arrest in a one-step coating process. Generally, with the more conventional high temperature cementation packs, it is not uncommon to place a charged retort into a hot furnace in order to speed up the production rate.

FIG. 4 shows the comparison of heat up rates for a retort placed in a cold furnace then raised to temperature and a retort placed into a furnace already heated to the desired temperature. The greater heat capacity of the hotter furnace produced a condition where the temperature arrest at 350 F. was either eliminated or suppressed (note curve F). This undesirable condition is considered to be the result of excessive decomposition of the aluminum chloride to products including aluminum oxide. The importance of this arrest at 350 F. in terms of the coating produced has been confirmed by metallographic sectioning of specimens. Fast heat up produced minimal case depth due to improper cleaning, whereas, a satisfactory coating thickness resulted when the retort was heated up at a slower rate.

It will be clearly apparent from FIG. 4 that the heating cycle of dotted curve F from ambient temperature to about 300 F. corresponds substantially to a heating rate of about 16 F. per minute, at which rate the endothermic arrest was completely suppressed or rendered ineffective. On the other hand, the heating cycle of the solid curve within the invention exhibits a slower heating rate from ambient temperature to 300 F. of about 6.5 F. per minute to produce an effective endothermic arrest E having a dwell time of approximately 35 to 45 minutes.

The dwell period of the endothermic arrest at substantially the sublimation temperature should be suflicient to allow for cleaning of the part to be coated before the part reaches the predetermined coating temperature (e.g. 750 F. to 900 F.). Thus, the dwell period of the endothermic arrest at the sublimation temperature should preferably be over about 15 minutes. Generally, an adequate dwell period is obtained at heating rates from ambient temperature to 300 or 350 F. of below about 8 F. per minute. Heating rates of over 9 R, such as 16 F. and 32 F. per minute, have been observed to cause suppression of the endothermic arrest for the purpose of the invention.

The necessary aluminum chloride addition to the pack has been found to range from about 1% and up to about The reuse of the coating compound only requires an aluminum chloride addition. The 1% aluminum chloride level is a preferred minimum requirement, but 2% is normally employed.

The mechanisms which are believed to explain the effect of the aluminum chloride in production of aluminum coatings are as follows, it being understood we are not to be held to this explanation:

The aluminum chloride has three functions in the pack. The first involves the cleaning action which takes place on the surface to be coated on the aluminum powder surface and also on the walls of the container holding the pack. This reaction is dependent on the availability of dry compound and dry aluminum chloride. In the case where quantities of moisture are present, undesirable room temperature reactions will occur which involve the generation of hydrochloric acids, the formation of hydrogen chloride vapors, or hydrolysis to a basic aluminum chloride compound. The effectiveness of the chloride cleaning is especially important when coating temperatures are to be in the region of 800 F. and is a function of the action obtained during the heat up cycle 'in the 350 F. region. This corresponds to the melting point or volatilization temperature of the cleansing and transport-inducing agent. In this case, it is believed that the AlCl removes metal surface layers even from 13% chromium steels. In this way, it is possible to prepare the surface of chromium bearing steels for the subsequent coating cycle. Surface contamination in many cases is also removed in the form of distillates by the reaction with the aluminum chloride. Further evidence of the cleaning action was obtained by comparing the coating extent, as a function of heat-up rate. One retort was placed in the furnace and given a normal heat up rate while a second retort was placed in a furnace where the heat up rate was at least twice as slow as the first.

10 All other conditions were identical. The arrest in the heat up rate, as indicated in FIG. 4, was noted only for the slowly heated retort. After the coating cycle, weight gains were all greater for the slow cycle. The results are listed below:

The foregoing effect can be explained by the eificient cleaning available because of the prolonged time the chloride vapors were available from the sublimation and melting process at below the metal coating and diffusion temperature. In cases where the heat-up rate was too rapid. only a partial cleaning resulted, and this was reflected in the lower coating thickness obtained. The importance of the cleaning action is especially of prime consideration in 13% chromium steels and other alloys which form passive layers when exposed to air. This layer must be removed to provide a fully compatible surface for coating. The absence of passive layers in chromium free steels allows substantial coating thicknesses to be formed in comparison to stainless materials in which the passive layer inhibits metal diffusion.

The cleaning activity of the pack was also substantiated by studying the effects of various additions. Urea NH CONH which decomposes to form an ammonia and a carbon dioxide atmosphere completely inhibited the coating process and a dark discolored surface resulted. Ammonium chloride also eliminated the formation of the coating.

Indications are that numerous reactions occur in the aluminum coating process. A number of these are listed below but other chemical processes may be significant. For example, the distillates formed by aluminum chloride, oils and residues on the surface of the steels may strongly influence coating.

Any moisture present on the compound Will tend to form the hydrated chloride which is believed to undergo decomposition as follows: AlCl .6H O decomposing to aluminum oxide and hydrochloric acid. The compound in the pack will undergo reactions as it is heated up toward the sublimation temperature. The main one is believed to involve the moisture present and aluminum chloride as follows:

Too rapid a heat up rate of the aluminum chloride will tend to produce excessive decomposition to HCl and the A1 0 products.

The desired reactions can be described as follows: Al Cl l+H O+Oxygen giving products of the oxy-chloride type [AlOCl] plus aluminum oxide and hydrogen chloride. The oxide formed will depend on the available oxygen and may initially be of the A10, A1 0 form but, because of the oxygen level in the pack, A1 0 will be the the final product. A compound analogous to HAlCl may probably also exist.

As the temperature of 825 F. is approached the dimer chloride will decompose to the monomer. This reaction is 335F AlzCls 2A1C1a At the coating temperatures, the aluminum vapor coating potential is derived from the following reactions:

HzO AlCl A101 ZHCI 1 1 The aluminum chloride AlCl will react in the presence of the aluminum particles to give the following coating potential:

A (pack) Vapor zAlCl 2Al T on As will be apparent from the foregoing description, the explosibility hazard occasioned by the use of fine (325 mesh) aluminum particles in the conventional pack cementation process can be eliminated by using coarse particles. The advantage of using a relatively coarse powder pack is that the powder charge is free flowing and allows the coating of complex shaped parts by eliminating packing difficulties. This is particularly true in the case of a vane and shroud assembly having a plurality of narrow channels.

While the invention has been described with specific reference to aluminum chloride, it will be apparent that numerous other cleansing and transport-inducing agents can be employed. The following systems were tested using a pack mixture containing 90% Al and 10% aluminum oxide to which were added separately 2% by weight of the pack one of each of the following:

Bismuth triiodide Bismuth chloride Ferric chloride Nickel iodide Titanium tetraiodide Aluminum chloride Aluminum bromide Aluminum iodide Cadmium iodide Cobalt chloride Nickelous chloride Tungsten hexachloride Lead iodide Trichloracetic acid Cadmium chloride MgO Iodine Ammonium bromide Ammonium chloride Ammonium iodide Aluminum chloride (hydrated) Example 1 About 800 lbs. of -60+140 mesh aluminum is mixed with 200 lbs. of A1 of also 63+140 mesh size. To the 1,000 lbs. of pack of material, 30 lbs. of dry AlCl is added under a humidity not exceeding 45 Generally, the humidity for packs should be less than 60% and, most advantageously, below 45 The pack is mixed in a vibrating blender for from about 5 to minutes. If the charge is a fresh charge, it is subjected to burn-out at 795-825 F. for 36 hours. However, if the charge has been previously used, burn-out is not required. The pack is placed in a dry condition in an aluminum lined retort with the parts to be treated, such as compressor blades (e.g. AMS 5616 steel), completely embedded in the pack. The cover is sealed to the retort body with multiple layers of aluminum foil in the form of a gasket suflicient to prevent air from getting in but to allow outgassing of gaseous by-products.

The retort is placed in an oven at ambient temperature and the temperature allowed to rise to the desired coating temperature by the application of heat. As the temperature rises, it goes through an endothermic arrest at about 350 F. and then allowed to reach a range of about 795825 F. and the retort maintained at substantially that temperature range for about 36 hours.

Upon completion of the heating cycle, the retort is removed from the oven and allowed to cool approximately 400 B, after which it is placed in a dry environment for cooling to ambient temperature.

The cooled retort is then placed in a humidity control cabinet the cover removed and the parts removed from the cementation pack. The parts are then cleaned of adhering coating compound by blowing with dry air and immersed in water to remove fine dust and other residues. The compressor blades exhibited a very clean deposit of aluminum.

Example 2 The method of Example 1 is repeated except that steel parts made of 17-4PH stainless (17% Cr, 4% Ni, 3% Cu and the balance essentially iron) are coated with zinc. The pack is carefully prepared as described in Example 1 except that it is comprised of 60% by weight of 40+140 mesh zinc, 40% by weight of A1 0 to which is added and blended dry zinc chloride in an amount of 3% by weight of the foregoing pack composition. Substantially the same heating cycle is employed as Example 1 except that the retort is maintained at a temperature of about 775 F. for about 24 hours. The steel part produced in this manner will have a uniform and clean coating of zinc covering the surface thereof.

Example 3 Managing steels have excellent high strength in the age hardened condition. A nominal composition of one such steel is 18.5% Ni, 8.5% Co, 3.5% Mo, 0.03% C, 0.25% Ti, 0.15% Al and the balance essentially iron. This steel is substantially fully hardened by heating it at a temperature of about 900 F. for about 3 to 6 hours. Unfortunately, the steel does not have desirable resistance to oxidation at elevated temperatures. The method of the invention may be employed using an aluminum-A1 0 pack of the type described in Example 1 except that 3% by weight of dry ferric chloride is added to the pack as the cleansing and transport-inducing agent. The same heating cycle as that of Example 1 is employed except that the part is heated to 850 F. and maintained at that temperature for about 36 hours. This process produces a uniform coating comprised substantially of aluminum which markedly improves the resistance of the maraging steels to temperatures of up to 900 F. while maintaining substantially its original hardness.

Example 4 Heat resisting alloy articles may likewise be treated by the invention. A nickel-base superalloy known by the trademark Inco 700 containing 45% Ni, 15% Cr, 30% Co, 3% M0, 3.2% A1, 2.2% Ti, 1% Fe, 0.13% C and the balance residuals, such as Mn, Si, etc., is illustrative of such alloys that can be treated. Using the same heating cycle of Example 1 and the same cementation pack composition, an adherent aluminum coating may be produced using a final temperature of 1100 F. for 24 hours.

One of the advantages of the invention is that the cementation pack may be stored under conditions of relatively low humidity, e.g. below 60% or 45% humidity in plastic bags. One such composition comprises 60 to aluminum, and the balance alumina over a particle size range of 60 to mesh, the dry aluminum chloride added to the mixture ranging from about 1% to 5% by weight of the pack.

While the invention has been particularly described with respect to protecting chromium-containing steels, such steels may contain about 5 to 25% chromium, up to about 5% tungsten, up to about 5% nickel, up to about 4% copper, up to about 3% aluminum, up to about 2% titanium, and the balance essentially iron, and include, in particular, precipitation hardenable chromium-containing stainless steels. It will also be appreciated that the invention may be employed on iron-base, nickel-base and cobalt-base alloys, refractory metals, such as tungsten, mo-

13 lybdenum, titanium, and the like. The term metal base or base metal article is meant to include the foregoing metals and alloys.

As has been pointed out hereinbefore, a particular advantage of the invention is the relatively low temperature range over which high quality coatings can be achieved. Such low coating temperatures can range from about 750 F. to 900 F., and, more advantageously, from about 775 F. to 850F. Examples of coating metals which may be employed in caring out the invention in addition to aluminum are Cd, Zn, Sb, Cu, Fe, Mn and In, among others.

It will be understood that while the invention has been described in its details, specifically with reference to preferred embodiments thereof, changes and modifications can be made without departing from the spirit and scope of the invention.

For example, the invention provides as one of its embodiments, a metal product characterized by improved resistance to failure by fatigue coupled with improved resistance to corrosion and oxidation. As has been stated herein, the invention is particularly applicable to the production of structural elements or components made of chromium-containing steels for aircraft, which elements in service are subjected to thermal shock, vibration, centrifugal force, and the like, at temperatures of upwards of 850 F. or 900 F. Examples of structural elements produced from chromium-containing steels are turbine vanes, shrouds, compressor blades, casings, and the like. In order to insure adequate resistance to failure by fatigue, the elements are shot peened to build up compressive stresses in the substrate. As pointed out in FIG. 1, shot peening has a marked effect on upgrading the resistance of structural elements to fatigue, even where the element in use has already been corroded by the environment as illustrated by curve C of FIG. 1. As will be noted, shot peening upgrades the fatigue resistance to the marked- 1y high level of curve B, which resistance is further optimized by thermally coating the substrate with aluminum as shown by curve A due to the cleaning action of the aluminum coating process, particularly in providing a clean interface between the substrate and the coating. Thus, the invention enables reworking and upgrading prevrously used structural elements and parts, such as compressor blades, nozzle vane shrouds, etc., to the extent that they compare favorably in properties to new parts. Thus, summarizing the foregoing, the invention provides as an article of manufacture, a thermally aluminumcoated structural element of chromium-containing steel having a coating comprising a heat diffused layer of aluminum 1n the substrate thereof, said substrate being characterized by compressize stresses introduced by shot peenmg, the compressive stresses at the substrate, which may extend up to about 0.01 inch in depth, together with the overlying diffused layer of aluminum, providing improved resistance to fatigue coupled with improved resistance to corrosion and oxidation at temperatures up to about 850 F. or 900 F.

Although the present invention has been described in COIIJUDCUOII with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

1. In a method for diffusion coating aluminum into the surface of a chromium-containing steel article by pack cementation, wherein the article is embedded in a cementation pack and heated to an elevated temperature to effect diffusion of aluminum in the steel article, the improvement of diffusion coating said article at a temperature being below the temperature which is detrimental to the physical properties of said steel which comprises,

t 14 providing a cementation pack comprised of particulate aluminum, said cementation pack having uniformly mixed therewith an amount of dry aluminum chloride as a cleansing and transport-inducing agent efiective to promote the deposition of aluminum on said article at said elevated metal coating and diffusion temperature, said aluminum chloride being characterized by substantial vapor pressure when heated to its sublimation temperature below said metal coating and diffusion temperature, completely embedding the steel article into the cementation pack confined in a retort maintained at a relative humidity not exceeding about 60%, subjecting said pack and the embedded article to a heating cycle by heating said pack to said metal coating and diffusion temperature at a rate to produce an endothermic arrest at substantially said sublimation temperature for a period of over about 15 minutes effective to clean the surface of said article, with substantial outgassing occurring during said endothermic arrest, and continuing said heating to and at said metal coating and diffusion temperature in the range of about 750 F. to 900 F. until the desired aluminum coating thickness has been obtained.

2. The method of claim 1, wherein the pack contains by weight about 1% to 5% of dry aluminum chloride.

3. The method of claim 2, wherein the pack to which the aluminum chloride is added contains about 60% to aluminum and the balance a stable inert refractory oxide.

4. The method of claim 3, wherein the inert material is alumina, and wherein the particle size of the pack material exceeds 200 mesh.

5. The method of claim 4, wherein the particle size of the pack material ranges from about -60 mesh to mesh.

6. A method of diffusion coating aluminum into the surface of a steel article at a temperature below the temperature which is detrimental to the physical properties of said steel which comprises, embedding said article in a pack comprised of particulate aluminum having mixed therewith about 1% to 5% dry aluminum chloride, said pack being maintained at a relative humidity not exceeding about 60%, subjecting said pack and article confined in a retort to heating in which the confined pack is held at substantially the sublimation temperature of the aluminum chloride for a period of over about 15 minutes effective to clean the surface of said article, and continuing said heating to and at a predetermined metal coating and diffusion temperature in the range of about 750 F. to 900 F. until the desired aluminum coating thickness has been obtained.

7. A method of diffusion coating aluminum into the surface of a steel article at a temperature below the temperature which is detrimental to the physical properties of said steel which comprises,

providing a cementation pack comprised at particulate aluminum having a dryness corresponding to a relative humidity of less than 60%,

mixing with said pack an amount of dry aluminum chloride maintained at a dryness also corresponding to a relative humidity of less than 60% effective to promote the deposition of aluminum on said steel article,

completely embedding said steel article within said pack in a sealed retort while maintaining said pack at a relative humidity below 60%,

subjecting said pack and article confined in said sealed retort to heating in which the confined pack is held at substantially the sublimation temperature of the aluminum chloride for a time effective to clean the surface of said article,

and then continuing said heating to and at a diffusion temperature in the range of about 750 F. to 900 F. until the desired aluminum coating thickness has been obtained.

(References on following page) 16 References Cited ALFRED L. LEAVITT, Primary Examiner UNITED STATES PATENTS M. F. ESPOSITO, Assistant Examiner 3,096,160 7/1963 Puyear 117107.2 P 3,257,230 6/1966 Wachtell at a]. 117 101.2 P 3,254,969 6/1966 Bungardt 117107.2 P X 5 4I5200; 416 241 3,325,305 6/1967 Schley "117107.2 P 

